If your soil is not likely to respond to added fertilizer, the question of "how much yield will you get if you add fertilizer" does not arise. However, if the probability calculated in the previous step suggests that added fertilizer will have some benefit, then we need to estimate how much benefit might we expect, in terms of additional bushels per acre. This additional production is called Delta Yield.

Microbes either uptake nitrogen when their environment is carbon rich in a process called immobilization, or release nitrogen in the form of plant-available nitrate (NO₃^- in the process of mineralization. The carbon to nitrogen ratio and the soil pH are important factors in this metabolic switch.

An economically optimum nitrogen rate (EONR) is a nitrogen rate that maximizes the returns on investments in nitrogen application. This rate depends on how responsive your soil is, how weather affected the growing season, current costs for various forms and applications of nitrogen and projected revenues from the yield. Calculating a true EONR prospectively is very difficult. However a simpler form of the EONR is often referenced in calculators like the Maximum Return to Nitrogen (MRTN).

Making nitrogen rate recommendations requires several pieces of information, including historic yields, management practices, soil types and of course current soil tests. In this second part we look at the three key soil tests needed to make good recommendations.

This is the first of a multipart series on how to make site-specific, nitrogen rate recommendations using the SoilDx FertiSaver-N system.